Lindsay Lohan was in New York while her ex-girlfriend Sam Ronson was hanging out with her ex-best friend Nicole Richie in L.A. (X17Online.com)

Nicole Richie had her second child, Sparrow, a month ago, but that hasn’t stopped her from going out and having a good time with friends.

On Friday night, Nicole and boyfriend/baby daddy Joel Madden went out dancing and drinking at Playhouse Hollywood with pal Samantha Ronson. Nicole and the gang did shots of lemon drops and danced together in the VIP room. At one point, Nicole coaxed Sam to get in the DJ booth and spin some of their favorite songs, even though Sam was off the clock.

Clubgoers tell Fox411 that Sam and Nicole were acting like best friends, a scene that put Lindsay Lohan over the edge once she heard about it.

“When Lindsay found out that Sam was out with Nicole, she was not happy!” an insider tells Fox411, explaining that Lindsay and Nicole, once best friends, have grown apart in recent years. “Since Lindsay’s return to court last week, she has to be extra careful for probation, and [hearing about] Sam out with Nicole hurt her.”

The friend explained the rift between Lindsay and Nicole, and why Sam is stuck in the middle.

“Nicole has made it very clear to Lindsay and to their mutual friends that she wants a better life for her. Nicole hopes Lindsay will get it together and change her behavior, but until she makes a change, Nicole avoids her,” says the pal. “Sam and the rest of the Ronson family have remained close to Nicole, Joel and his brothers.”

It sounds like Lindsay’s ex is trying in vain to stay above the fray.

“Sam loves Nicole and Joel and she cares deeply about Lindsay,” the source says. “She in no way wants to hurt her, but Sam will not drop her friendship with Nicole.”

Which makes Lohan crazy.

“Lindsay takes her hanging out with Nicole personally [and] as an attack on their on/off relationship,” says the pal.

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